Stratospheric Dynamics: Wave Measurements and Modeling

Leonhard Pfister and K. Roland Chan

Overview

The lower stratosphere is the region where significant ozone depletion has
occurred as a result of human activities (the prime example is the Antarctic
ozone hole). Chemical reaction rates in this region are slow; therefore,
determining the origin and properties of air motions in the lower stratosphere
is vitally important to understand the distribution of ozone depletion.
Waves, a particular type of air motion, drive the overall stratospheric
circulation and, thus, govern the global distribution of ozone.

The waves of greatest interest to this project have sizes ranging from a
typical house lot to 100-200 miles. Too small to be resolved by satellites,
their properties are detected by instruments on high-flying aircraft,
such as NASA's ER-2. Two of the most important causes for these waves are
depicted in the illustrations on the reverse. Figure 1 shows waves excited by
air flow over mountains, as measured by the Ames Meteorological Measurement
System (MMS) aboard the ER-2. Like ocean waves, these "mountain" waves are
able to travel substantial distances. In this case, they travel upward about
12 miles to
the flight level of the ER-2. Upon reaching this level, the mountain wave
interacts with the flow to produce a long string of wavelets following the
main wave at 17!E. The details of this interaction and its significance for
the overall stratospheric circulation are currently under study.

Figure 2 shows waves excited by convective clouds, in this case a tropical
cyclone (hurricane) off northern Australia. Here, dark green and black denote
regions with the highest clouds and strongest convection; yellow and red
areas indicate weaker convection. The white contours depict the structure of
the wave, as determined by a model simulation based on ER-2 MMS measurements
made above the highest clouds. The wave is similar in many ways to the waves
caused by a pebble thrown into a stream, with the pebble being the hurricane
and the stream being the stratospheric flow (in this case, from right to
left). These waves drive the overall circulation at all altitudes in the
tropical stratosphere. Model calculations show that they are partially
responsible for the vertical flow between the lower atmosphere (troposphere)
and the stratosphere.